1. Field of the Invention
The present invention relates to the field of densitometry, and more specifically, to a densitometer which automatically aligns itself with a sample to be measured.
2. Description of the Prior Art
A reflection densitometer is a well known optical measurement instrument that is used for quality control in color printing production processes. The most common use of a reflection densitometer is to measure ink on paper to control the printing process, although densitometers are also used to measure photographic prints and other images on various substrates such as paper, plastic and metal, in order to monitor and control the production of the images or to test the production equipment.
Basically, a reflection densitometer directs a beam of light having a known color quality, i.e., a balance of red, green and blue light in conformance with ANSI/ISO standards, at a printed sample. By using a filtration system, the densitometer then measures the amount of light which is reflected from the surface of the sample and generates an output signal which is indicative of the color density of the sample. In practice, it is most common that the densitometer output signal is actually the reflectance of the sample, but a conversion to density is straightforward since density is the inverse logarithm of reflectance. Unless otherwise indicated, it will be assumed herein that a densitometer generates a reflectance signal as its output.
Illumination and measurement systems are referenced to an axis which is perpendicular to the sample. According to accepted standards, the illumination angle is either 45.degree. or 0.degree. degrees, and the measurement angle is then 0.degree. or 45.degree., respectively. It is then assumed that the light absorbed by the sample is the difference between the incident light and the reflected light. The sample color density can then be calculated according to an accepted logarithmic relationship.
Automatic densitometers, such as the AutoSmart.TM. Densitometer made by Graphic Microsystems, Inc., have been developed which use a computer to control the position of the densitometer with respect to a measurement table, thus enabling the densitometer to be automatically driven to measure a predefined pattern of measurement points on the printed sample. Using motor driven horizontal and vertical positioning, automatic densitometers can read up to 250 points on a printed sample in less than one minute.
However, it is difficult to place a printed sample sheet in an identical x-y orientation relative to the measurement table every time a sheet is placed on the table for measurement. Thus, the position of each measurement point on a sample sheet may be slightly offset relative to the table for each successive sheet which is measured. This causes a conventional automatic densitometer measurement scheme to take readings at close, but not exact locations. Therefore, it is desirable to have a method of locating the pattern of measurement points with a high degree of accuracy each time a new sheet is put on the table for measurement.
The AutoSmart.TM. Densitometer requires an operator to manually align an optical target in the densitometer with two or three selected "alignment points" on the printed sample. Once the positions of the three alignment points are known, the locations of all points to be measured can be calculated by the computer.
Some automatic position indicators use auxiliary targets and sensors to locate the targets. For example, the Hammamatsu quadrant detector can be used to locate targets such as light spots. Bar code scanners can be used to sense the widths of target lines. Other more elaborate schemes use "machine vision" techniques that locate targets by using video images. The disadvantage of all of these methods is that they require complex and costly additional equipment, including a light source, a detector, and electronic processing circuitry.
If the task of locating the alignment points could be automated, the measurement of a sample sheet would be easier and faster. Also, the accuracy of locating the measurement points, which was previously limited by the operator's ability to visually position the target on an alignment point, can be improved.